It shouldn't be... but then again knowing Ryanair, and the way they use ANY excuse to charge passengers extra fees, I wouldn't be surprised if they took the attitude "It's not A4 - the rules say A4" and charge you for a replacement.

So - if you can't get A4 paper you may want to take precautions by taking a PDF copy with you (in addition to a paper copy on US Letter) and trying to get an A4 print in Europe.

As for other tips... I presume you are already aware of all the standard things that one needs to know like "strictly one item" (they've been known to kick up a fuss about water bottles or books carried in people's hands).

I would recommend taking some music and a good pair of noise-cancelling headphones as the in-flight experience can be very noisy and irritating (one commercial after the next)

Personally I avoid Ryanair (because of their attitude) but I have used them in emergencies and they are bearable - just.